1. Field of the Invention
The present invention relates to a control system for optimizing instantaneously (in real time) the working point of a four-stroke internal-combustion engine through real time processing of parameters indicative of the running thereof, such as the pressure prevailing in the various combustion chambers at a series of successive times of each combustion cycle, the engine speed, etc, and for controlling optimum fuel injection, ignition, valve timing adjustments, etc.
The invention may be used in research laboratories to design control systems for automobile engines and may be integrated in the control systems of high-performance engines within a vehicle.
2. Description of the Prior Art
It is well-known to associate one or several detectors with an engine for measuring several parameters so as to control the running thereof better. These detectors may be accelerometers for detecting knocking, pressure detectors for measuring the pressure inside the chambers or in the induction and the exhaust pipes, temperature detectors, injection needle lift detectors, etc.
It is also well-known to apply predetermined adjustments to an engine by means of a computer, by relating the indications provided by the detectors to stored type configurations. Many detectors are necessary and complex type configuration charts relating to very diverse running conditions have to be "mapped" in order that this process is efficient.
Control may be improved if in real time the values of some complex parameters representative of the instantaneous running conditions of an engine are defined.
This is notably the case of the mean indicated pressure (PMI) which provides a direct indication on the useful work of the gases during a combustion cycle and allows comparisons to be drawn between different engines.
It is also the case of another parameter ST which defines the stability in time of the PMI and provides information on the combustion regularity.
It is known that, in a two-stroke or a four-stroke internal-combustion engine, the useful work from the gases (or indicated work) for a cycle may be obtained by measuring (for example by plotting) the area of a diagram P/V (FIGS. 1A, 1B) recorded for the engine, or by computing on an engine cycle the relation: EQU W=.intg.pdV=.intg.Fdl=.intg.pSdl (1)
where F is the force exerted by the gases on each piston, p is the pressure prevailing in the combustion chamber, dV is the volume variation of the chamber, dl is the corresponding displacement of the piston and S is the projected surface thereof.
The PMI is defined as the constant pressure which would provide the same indicated work Wi per cycle when applied to the piston during the total expansion stroke thereof: EQU Wi=PMI.S.1
The PMI is obtained, as it is known to specialists, through the relation: ##EQU1## where P(V) is the pressure measured as a function of volume V, V.sub.T is the volume swept by the piston during its travel between the bottom dead center PMB and the top dead center PMH.
In FIG. 2, the parameters referred to therein are: .alpha. the angle formed by crank 4 with the axis of cylinder 1, B the angle formed by the length of connecting rod 3 and the axis of the cylinder, a the offset of piston 2 with respect to the axis of crankshaft 5, m/2 the length of the crank 4, and V(.alpha.)the volume variation of the combustion chamber when the crank sweeps angle .alpha. up to the top dead center.
By expressing the variation in volume as a function of .alpha., relation (2) may also be expressed as follows: ##EQU2##
A simple calculation shows that, by taking the previous designations into account, the previous expression may be written in the following form: ##EQU3## where h-sin .beta.=a/B+m sin/2B.
The stability factor ST of the value PMI is obtained from a collection of n values PMI successively obtained and stored (n=100 or 1000 for example), and it is established by applying the following relation: EQU ST=standard deviation of PMI/mean value of PMI,
this factor being established from the collection of values obtained.
Knowledge of the values PMI of an engine is for example used for determining the optimum advanced ignition AA.degree. allowing predetermined criteria to be met.
A first criterion may consist in obtaining a minimum indicated specific fuel consumption (I.S.C.) or a maximum engine torque.
A second criterion may consist in minimizing the most polluting fractions of the burnt gases such as Nox. The optimum advanced ignition AA.degree. selected depends here directly on the value found for the stability factor ST.
A third criterion may for example consist in ensuring the maximum stability of the engine torque.
To perform these calculations, it is possible to use a computer programmed to compute the PMI and the factor ST values from a set of discrete pressure measurements obtained at successive times of a single cycle. Computations may be carried out in real time simultaneously for all the cylinders, but, the following criteria are used
A relatively low sampling frequency of the measured pressures is selected, which leads to rather approximate values of the previous pressure PMI and factor ST
Alternatively, the pressure samples measured in the various cylinders have to be acquired successively by multiplexing, which leads to decreasing the number of values PMI obtained.
In both cases, the limitations imposed by insufficient acquisition and computing speeds adds considerably uncertainty to the actual values PMI and on their stability factor ST, which reduces the possibilities of an efficient running control of engines, and interferes with the development of control techniques, notably if vehicles are to be equipped with such systems.